Generally, an electronic ballast is used to provide an AC (alternating current) voltage to drive gas discharge lamps, such as fluorescent lamp, high pressure sodium lamp and metal halid lamp. An input voltage is derived from an AC power supply or battery and converted into a DC (direct current) input voltage. The DC input voltage is then converted into an AC driving voltage by an inverter.
Different driving voltages are needed during different operating phases. A driving voltage, which may be variable from hundreds to ten thousands of volts based on the characteristics and application of the lamp, is needed during ignition. However, after the lamp being ignited and entering into the steady state, the operating voltage across it is much lower, such as 200 volts.
Two igniting methods are commonly used. One is resonance igniting method, wherein the ignition voltage is generated by a resonance circuit with frequency sweeping. The other is pulse igniting method, wherein a high voltage pulse signal is generated by a switch and a coupled inductor to ignite the lamp. If the ignition fails, the ballast may stop working or try to ignite the lamp again after a certain time period.
FIG. 1 is a block diagram of a prior ballast using pulse igniting method. It comprises a voltage converter 101, an inverter 102, a pulse generator 103, an inductor L, a coupled inductor Lcouple and a switch Sstrike. The voltage converter 101 receives an input voltage Vin from an AC power supply or battery and converts it into a DC input voltage Vdc. The voltage converter 101 may comprise a rectifier bridge, a DC/DC converter or an AC/DC converter. The inverter 102 is electrically coupled to the voltage converter 101, receives the DC input voltage Vdc and generates an AC driving voltage Vout across the lamp through the inductor L. The inverter 102 may utilize any DC/AC topology, such as full bridge, half bridge and so on. The coupled inductor Lcouple is magnetically coupled to the inductor L. One terminal of the coupled inductor Lcouple is electrically coupled to receive the DC input voltage Vdc. The switch Sstrike is electrically coupled between another terminal of the coupled inductor Lcouple and the ground. The pulse generator 103 is electrically coupled to the gate of the switch Sstrike, generates an ignition pulse when the ballast is started up. The switch Sstrike is turned on for a time period and then turned off by the ignition pulse, so a high voltage is generated across the inductor L. This voltage is applied across the lamp to ignite it.
FIG. 2 is a block diagram of a prior ballast using resonance igniting method. It comprises a voltage converter 201, an inverter 202, a frequency sweeping circuit 204, an inductor L, capacitors Cs and Cp. The voltage converter 201 and inverter 202 are similar to the corresponding circuits in FIG. 1. The capacitor Cs is serially coupled to the inductor L. The capacitor Cp is electrically coupled to the lamp in parallel. A resonance circuit is formed by the capacitors Cs, Cp and the inductor L. The inverter 202 comprises at least one switch. The frequency sweeping circuit 204 is electrically coupled to the inverter 202. When the ballast is started up, the switching frequency of the inverter 202 is reduced by the frequency sweeping circuit 204 from a value which is larger the resonance frequency of the resonance circuit. So a high voltage is generated across the lamp to ignite it.
In the igniting methods mentioned above, the ignition voltage is directly related to the DC input voltage Vdc. The larger the DC input voltage Vdc, the larger the ignition voltage. When the ballast is just started up, the DC input voltage Vdc is unstable. It may be much larger or smaller than the predetermined value, which will cause the ignition voltage to be too high or too low. The ballast and lamp will be destroyed if the ignition voltage is too high. The lamp won't be timely ignited if the ignition voltage is too low.